The present invention relates to a structured latex. More particularly, the present invention relates to such a structured latex that is particularly adapted for use in adhesive applications having one polymeric phase adapted to provide adhesive properties and another polymer phase adapted to provide tackifying properties. More particularly still, the present invention relates to such a structured latex wherein both the adhesive and tackifier phases are prepared by the emulsion polymerization of at least one of the phases in the presence of the second phase (in-situ tackifying).
Aqueous collodial dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful as components of adhesive formulations, as well as binders and coatings. A wide variety of adhesive formulations have been prepared that are specially adapted for particular uses. While latexes generally provide good adhesion to various substrates, it is often desirable to include a second component, generally a low molecular weight emulsified resin that is employed in order to impart initial stickiness or tack to the adhesive formulation. Such a component is referred to as a tackifier. Certain prior art adhesive formulations have included latex based adhesive components such as a latex of a styrene/butadiene copolymer, and conventional tackifiers such as rosin esters, or other suitable tackifiers, See, e.g., U.S. Pat. No. 4,189,419. The properties of the adhesive composition may be adjusted by appropriate selection of both the type and amounts of tackifier or adhesive component employed in order to provide a resulting composition having the desired properties.
However, such adhesive formulations have certain undesirable properties. For example, the process for dispersing the tackifying resin adds unnecessary complexity and additional cost to the formulation. In addition, such formulations often demonstrate insufficient coalescence between resin particles and latex particles thereby necessitating using excessive amounts of tackifier resin. This problem has been previously disclosed in U.S. Pat. No. 4,445,959. Also, because relatively high levels of surfactant are required to disperse the tackifying resin, adhesive properties are often detrimentally affected. Finally, under some conditions of high shear such as may be encountered during application of adhesives, these previously known adhesive formulations may lack sufficient stability.
As far as is known to the present inventors, it has previously been unknown to prepare, in a single latex composition, the desired combination of adhesion and tack properties to result in a suitable adhesive formulation, despite the obviously desirable processing and cost advantages to be gained from such a preparation. Attempts to prepare such a single latex composition have not previously succeeded due to the fundamentally opposite characteristics embodied in the adhesive phase and the tackifier phase of such latex. Generally, latexes which represent a compromise in adhesive and tackifier properties fail to possess adequate performance in either category. Consequently, previous attempts to prepare single component latex adhesive/tackifier compositions have proven unsatisfactory.
It would be desirable to provide a latex for use in preparing an adhesive formulation, especially a pressure sensitive adhesive, which does not require the additional presence of a conventional tackifier resin. It would, in addition, be desirable to provide such a composition which may be prepared entirely by emulsion polymerization techniques (in-situ tackified structured latex adhesive). Finally, it would be desirable to provide an in-situ tackified structured latex adhesive which possess performance as a pressure sensitive adhesive that is as good as or better than the performance of formulations containing latex adhesives and conventional tackifying resins.
Techniques are already known for the preparation of low molecular weight polymers via free-radical, emulsion polymerization processes. Typically, such techniques include initiation by organic peroxides or inorganic persulfate compounds, and the use of large quantities of a suitable chain transfer agent such as a halogen compound, a mercaptan compound, or the like, to limit growth of the polymer chains.
Structured latexes, also known as heterogeneous polymer latexes, or sequential emulsion polymerization latexes, have been previously described in U.S. Pat. Nos. 4,478,974; 4,537,916; 4,134,872; and 4,325,856. In such compositions, latex particles having domains exhibiting different properties such as "soft" and "hard" polymers, high strength or film forming properties, or hydrophobic and hydrophilic properties are prepared. The process used in the preparation of the above sequential emulsion polymerization is referred to as multi-stage emulsion polymerization.
For the teachings contained therein, the above-identified U.S. patents are hereby incorporated in their entirety by reference thereto.